The present invention relates to a rotary drum allowing countercurrent leaching of solids by a liquid for the purpose of extracting one or several components, in particular, the sucrose of sugar beets or sugar cane.
There are known various devices of this type including a rotary drum having a horizontal or slighly inclined axis, wherein the solids and the liquid circulate in opposite directions owing to a suitable structure, e.g. such as that described in Belgian Pat. Nos. 367,630 and 371,926. This known apparatus consists of a horizontal, large diameter drum which is divided into successive compartments by a helicoidal inner partition and by an arrangement of screens and/or perforate plates situated in a diametral plane. The rotation of the drum about its axis brings about an axial translation of the liquid, which occupies the lower portion of the drum and follows the spiral by passing through the screens. However, on each half-turn of the drum, the solids are stopped by the screens, are lifted by the screens to the upper portion of the drum and are made to glide by the screens into the preceding spiral turn through chutes arranged on the diametral plane in directions opposite to the direction of the spiral. Accordingly, during each complete revolution, the solids advance twice by a half pitch of the spiral toward its upstream end. That is, the solids advance at the same velocity as the liquid but in the opposite direction. A feature of this apparatus is that the arrangement of the screens in a diametral plane, which cannot be traversed by the solids, requires the division of the drum into two portions that must be alternately fed with solids on each half-turn of the drum.
According to a later apparatus described in Belgian Pat. No. 475,626, the spiral containing the circulated liquid possesses a double inlet, which doubles the translational velocity of the liquid in relation to that of the solids. The time of stay of the liquid in the drum is reduced by half, which is useful when the liquid rapidly deteriorates, as in the case of sweet juices. Accordingly, in addition to the two partial circuits of the solids, the liquid in this apparatus is separated into two parallel flows that never mix. These known devices are associated with several disadvantages:
(a) The chutes wherein the solids must glide against the direction of the spiral readily become clogged. In order to avoid this, it is necessary to provide a substantial slope, which requires a drum of large diameter, and a moderate speed of rotation, thus limiting the capacity of the apparatus.
(b) The countercurrent principle cannot be perfectly realized because, due to the presence of two separate circuits of solids that are alternately immersed into each of the two flows of liquid, the solids of a given circuit contact the same flow of liquid only once every two turns of the drum.
(c) The separation of the liquid and the solids is incomplete because the solids are stopped by the diametral screens and/or perforate plates having the effect of a vertical portcullis, against which the solids are heaped in a compact mass that obstructs the passage of the liquid.
(d) The position of the screens and/or perforate plates in diametral planes and the presence of the chutes crowding the axial portion of the apparatus geometrically restrict the size of the surface of such screens and/or plates.
(e) When the diametral plane passes beyond the horizontal, the liquid which continues to drain from the emerged solids no longer is separated from the solids, but is drawn by the slope of the diametral plane and precedes the solids in the direction of their gliding movement.
In order to correct the above deficient separation of solids and liquid, certain known devices were provided with perforate plates extended along the wall of the drum and the lateral walls of the compartments, or even with perforate plates facing each other and forming "spikes" perpendicular to the axis of the drum, that divide the compartments in the direction of the width. In order to correct the clogging of the chutes, according to a known apparatus described in Belgian Pat. Nos. 711,219 and 728,417, the solids are made to follow the spiral and the liquid circulates in a direction opposite thereto in radial channels, while the diametral plane separating the two circuits of solids and the presence of the two parallel flows of liquid are retained. However, the feature of the rotary drum of these known arrangements with the inlet for the solids on the upstream side of the spiral and the inlet for the liquid on the downstream side of the spiral claimed in such patents was previously known, e.g. from USSR Author's Certificate No. 76,594. The apparatus described in Belgian Pat. Nos. 711,219 and 728,417 is associated with an additional disadvantage in that the solids lifted to the upper portion of the drum by the above mentioned diametral planes are no longer retarded by passage in the chutes and glide rapidly as a compact mass. Such compact mass is not effectively dispersed in the liquid and even frequently forms a heap or dome that is not submerged in the liquid.
One attempt to eliminate the disadvantages of the diametral plane is described in Italian Pat. No. 478,054. The screens are no longer arranged diametrally but along a chord. The screens are doubled by a solid plate which extends substantially parallel to the screens which are shifted by 45.degree. in relation to one another, so that the rate of displacement of the solids in relation to the drum amounts to seven-ninths of that of the liquid. This arrangement is associated with the grave disadvantage of unnecessarily complicating the transfer of liquid between the compartments, which transfer must be achieved by means of multiple pipe systems which are arranged outside the casing of the drum and which are liable to clog when solids are accumulated therein.
The above mentioned USSR Author's Certificate No. 76,594 describes a rotary drum having screens which are diametrally opposite but which have the shape of spring boards or steps on which the solids are lifted while the liquid traverses the screen, is dropped into the chamber situated below the spring board, is stopped by a solid wall and is forced back into the preceding spiral turn through an opening arranged in the wall of the spiral element. Under the effect of rotation of the drum, the solids then clear the spring board by gliding on the screens and drop beyond the solid wall into the lower portion of the drum that contains another fraction of the liquid. This apparatus is associated with the disadvantage that the height of the spring board, which determines the maximum level of the liquid, is insufficient because the spiral bands attached on the inner face of the drum leave open the axial portion thereof. Further, this apparatus will not prevent the liquid forced by inertia from clearing the spring board simultaneously with the solids. Accordingly, a drum of this kind can turn only very slowly with a low ratio of filling to volume, and therefore its productivity is insufficient for practical operation.
USSR Author's Certificate No. 135,425 describes an apparatus which is similar to the immediately above described known apparatus in principle, but which employs a double inlet spiral. The solids are lifted by plane screens whose sections perpendicular to the axis of the drum occupy positions of chords. The solids glide on the plane screens under the effect of rotation of the drum, while the liquid that traversed the screens is stopped by a solid wall and passes twice through the wall of the spiral element by means of radial ducts. In relation to the apparatus described in USSR Author's Certificate No. 76,594, this known apparatus possesses the advantage of better utilization of volume of the apparatus. However, the adoption of a double inlet spiral greatly complicates the structure, and the fact that the plane screens occupy a recessed position does not eliminate the above mentioned defects of the perforate diametral surfaces. Further, the gliding of the solids on the perforate surfaces should be avoided, due to erosion thus brought about and, especially in the case of sugar beet cossettes, due to damage to the cossettes and a resultant reduction in their permeability.